1. Field of the Invention
The invention generally relates to nonvolatile semiconductor memory devices, and particularly relates to a nonvolatile semiconductor memory device which stores electric charge in a nitride film.
2. Description of the Related Art
Some nonvolatile semiconductor memory devices utilize a nitride film as an electric charge capturing layer in a virtual ground array structure, thereby providing for a single memory cell transistor to store two-bit information. Such nonvolatile semiconductor memory devices treat the opposite ends of a single nitride film situated between two bit lines as two separate memory cells, and store two-bit data by controlling whether hot electron is injected into these two cells. This is made possible by the characteristics of the nitride film serving as an electric charge capturing layer in that electric charge does not move in such a film.
In nonvolatile semiconductor memory devices, generally, all the bit lines receive the same write potential at their drain node at the time of a write operation by which hot electron is injected. Further, the potential applied to the drain nodes stays the same whether a write-verify operation is performed or a data-read operation is performed.
The method as described above that stores two-bit information in the single electric charge capturing layer has a drawback in that the threshold of one cell is affected by the threshold of the other cell. Depending on whether one of the cells is in a programmed state or in an erased state, the threshold of the other cell varies. If a fixed write potential is used as in the conventional method, a threshold obtained after a write operation varies depending on the state of the other cell. The threshold of a cell after a write operation is higher when the other cell is in the programmed state than when the other cell is in the erased state.
This creates threshold variation among individual memory cells within one sector after the completion of a write operation. Such threshold variation causes variation in inter-band tunnel currents at the time of erase operations. This results in further threshold variation following erase operations, which brings about a delay in an erase time and the degradation of rewrite characteristics.
Accordingly, there is a need for a nonvolatile semiconductor memory device which reduces threshold variation following a write operation.